1. Field of the Invention
The present invention relates to a method for producing aromatic compound isomers. More precisely, the invention relates to a method of adsorptive separation for producing intended aromatic compound isomers by the use of a zeolite-containing adsorbent and a desorbent.
2. Description of the Related Art
Aromatic compounds are useful for starting materials for synthetic products and for intermediates for medicines and agricultural chemicals. For obtaining a specific isomer of an aromatic compound, known is a method of adsorptive separation that comprises contacting an isomer mixture with a zeolite-containing adsorbent.
For example, Japanese Patent Publication No. 45457/1989 discloses a method of using an adsorbent of faujasite zeolite for adsorptive separation of 2,6-dichlorotoluene; Japanese Patent Laid-Open No. 330025/1992 discloses a method of using an adsorbent of faujasite zeolite that indispensably contains potassium and lead ions, for adsorptive separation of m-dichlorobenzene; and Japanese Patent Publication No. 46933/1992 discloses a process of isomerizing halogenobenzene derivatives, in which HCl formed in the step of isomerization is first removed through distillation or the like, and a specific isomer is thereafter separated from the isomer mixture through adsorptive separation by the use of a zeolite adsorbent.
Regarding heterocyclic compounds, Japanese Patent Laid-Open Nos. 143162/1997 and 251230/1998 and EP 173,440 disclose a method of using an adsorbent of faujasite zeolite for adsorptive separation of a compound having a pyridine ring structure.
In the process of adsorptive separation of aromatic compound isomers by the use of a zeolite-containing adsorbent, the adsorbent is degraded while used continuously. In the process, therefore, the adsorbent used must be exchanged for a fresh one in good time or must be regenerated, for example, by burning it. Accordingly, a technique for prolonging the life or the regeneration cycle of adsorbent, if possible, will produce great industrial merits.
In the process of the above-mentioned Japanese Patent Publication No. 46933/1992 for adsorptive separation of an aromatic compound isomer by the use of a zeolite adsorbent, the step of removing the side product, HCl from the isomer mixture prior to the next step of separating the intended isomer by the use of the adsorbent is for preventing the adsorbent used from being degraded. However, the process is still not enough for satisfactorily prolonging the life and the regeneration cycle of the adsorbent to the industrial level.
In ordinary adsorptive separation of aromatic compound isomers, the desorbent used is generally circulated and reused. In the line, however, the desorbent circulated for reuse is often contaminated with minor impurities having existed in the starting aromatic compound isomers and with other impurities having formed through decomposition or reaction of the aromatic compound isomers and/or the desorbent, as the impurities accumulate in the desorbent being used and circulated. As a result, the adsorbent being thus used in the line will be gradually degraded.